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LIMESTONES AND MARLS                            143

tion is complete and the liquid cold (one drop of it should give no coloration
with either potassium ferrocyanide or thiocyanate), it is titrated with
standard permanganate solution. The amount of ferric oxide alone is
thus obtained.

8.    Lime  and  Magnesia.—In the filtrate from the aluminium and
ferric hydroxides, the lime is determined by acidifying faintly with hydro-
chloric acid, heating to boiling in a beaker and adding gradually a slight
excess of solid oxalic acid (about three times the supposed weight of the
lime and magnesia together).    Ammonia in excess is then added, with
stirring, the precipitate filtered off after some hours, washed with cold water,
dried, ignited in a platinum crucible, finally ifi a blowpipe flame, and weighed
as CaO.

In the nitrate from the lime the magnesia is estimated. To this end
the liquid is acidified with hydrochloric acid, evaporated if necessary to
about 200 c.c. and, when cold, precipitated in a beaker by addition of 40
c.c. of concentrated ammonia and sodium phosphate solution. After at
least 12 hours, the liquid is filtered and the precipitate washed with ammonia
solution (1:5) and dried, the filter-paper being burnt separately from the
precipitate and the whole ignited in a porcelain crucible. If the residue is
not white, it is treated with a few drops of nitric acid and again calcined,
the remaining magnesium pyrophosphate being weighed. Mg2P207 X
0-36207 — MgO.

9.  Sulphates.—In some cases limestone contains appreciable amounts
of sulphates (gypsum), which may be determined by dissolving a definite
weight (about 2 grams) in dilute hydrochloric acid, rendering the silica
insoluble in the usual way and removing it by filtration, and precipitating
with boiling barium chloride solution.   The liquid is left for some hours
on a water-bath and the precipitate filtered off, washed, dried and ignited :
BaS04 X 0-343 = S03.

10.  Sulphides.—If the limestone contains also sulphur as sulphides
(pyrites, etc.), this may be determined by dissolving a definite weight in
hydrochloric  acid  after  addition  of  a   little  solid  potassium chlorate.
The silica is then separated as before and the filtrate precipitated with
barium chloride.   The' excess of the barium sulphate over that obtained
as in 9 is derived from the sulphides :  BaS04 X 0-13738 = S.

11.  Other Determinations.—In rare cases, some other determina-
tions may be required.    Phosphoric acid, for instance, is estimated by
precipitation of the nitric acid solution with ammonium molybdate, as
with fertilisers.    Determination of the alkalies is scarcely ever necessary.


* *

The principal deductions drawn from the results of analysis of a limestone
or marl are based on the respective proportions of the principal components,
i.e., of calcium carbonate and clay (silica + alumina + ferric oxide). Accord-
ing to the content of clay, distinction is drawn between limestones, properly
so called, which contain only a minimal amount of clay ; argillaceous limestones,
in which 10 % may be present; and marls, which are described as calcareous,
with ic-25% of clay, as marls proper with 25-50% and as argillaceous marls,
with more than 50% of clay. With more than 80% of clay, the products mayineral Phosphates " (ibid., 1897, p. 663); G. Masoni: " Contribution to^me